Silicone rubber is characterized as having excellent heat resistance, weather resistance, release properties, and other properties, but has been defective in that the silicone rubber is inferior in breaking strength, water resistance, and gas impermeability as compared to general-purpose organic rubbers, or synthetic rubbers mainly composed of carbon skeleton.
Various attempts have, therefore, been made to blend silicone rubber with organic rubbers so as to obtain rubber compositions that retain the advantageous features of the two components. For example, such compositions so far proposed include those obtained by blending the two components merely mechanically as disclosed in JP-A-54-41957 (the term "JP-A" as used herein means an "unexamined published Japanese patent application), JP-A-55-139604, and JP-A-56-76444 and those obtained by blending the two components in such a manner that the polyorganosiloxane is bonded to carbon-carbon double bonds (--C.dbd.C--) in the organic rubber as described in JP-A-54-157149, JP-B-55-15497 (the term "JP-B" as used herein means an "examined Japanese patent publication), JP-A-55-31817, JP-A-56-76440, and JP-A-56-76441.
In practice, however, it is difficult to obtain uniform mixtures of silicone rubber and organic rubbers, particularly of silicone rubber and oil-resistant rubbers having polar groups, through kneading because of the poor compatibility between the two components.
Moreover, a blend of components that are not basically co-vulcanizable with one another gives vulcanized elastomers whose properties are exceedingly poor, even if each component can cures into an elastomer having excellent properties. Thus, the features of the two components have not yet been fully exhibited.